This invention relates to elements for sensing the oxygen content of a gaseous mixture, particularly the oxygen content of the exhaust stream from an internal combustion engine. More particularly, the invention relates to an oxygen-sensing element that is effective in measuring the oxygen content in exhaust mixtures resulting from the combustion of both fuel-rich and fuel-lean air-fuel mixtures. The sensing element can supply information to electronic control circuitry to control the content of an air-fuel mixture supplied to an internal combustion engine so that fuel consumption is minimized, pollutant emissions are limited and/or other goals are achieved.
Electronic controls responding to the oxygen content of the exhaust stream from internal combustion engines have been in use in production automobiles in the United States since about 1976. These controls assist automobile manufacturers in meeting pollution emission standards and fuel economy requirements. Most of the controls respond to information that is produced by a sensing element disposed in the exhaust stream. That element provides an electrical signal indicative of the oxygen content of the exhaust stream. When a so-called fuel-rich air-fuel mixture is supplied to an internal combustion gasoline engine, meaning there is an excess of fuel compared to the amount of oxygen available to burn the fuel, the exhaust stream contains little oxygen. When the air-fuel mixture is fuel-lean, there is an excess of air over that needed to burn the fuel, so the exhaust stream contains a relatively large amount of oxygen. At the point when the air-fuel mixture is precisely correct for complete combustion, i.e., the stoichiometric mixture, there is a transition in the oxygen content of the exhaust stream.
The oxygen-sensing elements most used and studied employ noble metal electrodes disposed on a zirconia body. In a device employing two opposed electrodes on a zirconia body with one of the electrodes exposed to a gaseous mixture containing a first oxygen content and the other electrode exposed to a gaseous mixture containing a different, second oxygen content, an EMF (electromotive force), i.e., the Nernst voltage, will appear across the electrodes. The magnitude of that EMF is related to the comparative oxygen contents, of the two mixtures, that is, to the comparative oxygen partial pressures of the two mixtures. If one of the mixtures is air, so that its oxygen content is known, the oxygen content of the other gas mixture can be determined.
Some of the known oxygen sensors employ the oxygen pumping phenomenon. Oxygen can be transported through solid electrolytes, like zirconia, when catalytic electrodes are disposed on the electrolyte. A voltage impressed across the electrodes will cause ionization of ambient oxygen at the negative polarity electrode. The oxygen ion will migrate through the electrolyte to the positive polarity electrode where the ion is neutralized and released from the catalytic electrode as an atomic or molecular species. That is, oxygen is transported or pumped under the influence of the electric field, from the vicinity of one electrode to the other. The volume of the oxygen pumped is measured by the electrical current flowing between the electrodes.
The Nernst voltage produced in a conventional oxygen sensor is proportional to the logarithm of the ratios of the oxygen partial pressures at the opposed electrodes. As a result, in a fuel-rich air-fuel mixture, meaning relatively low oxygen content in the exhaust stream, the Nernst voltage is relatively high, for example, hundreds of millivolts. However, for a fuel-lean air-fuel mixture, meaning increased oxygen content in the exhaust stream, the sensing element voltage decreases to less than about 100 millivolts. Moreover, the sensor sensitivity, i.e., the change in the sensor voltage in response to air-fuel variations in the fuel-lean mixtures, is relatively small. Thus it is difficult, with a conventional Nernst voltage sensor, to determine the air-fuel ratio from an exhaust stream produced from combusting a fuel-lean air-fuel mixture.
In order to achieve improved fuel economy, it is desirable to operate an internal combustion engine with a fuel-lean air-fuel mixture whenever possible, for example, when swift acceleration is not required. Since the conventional Nernst voltage sensor is ineffective in sensing the oxygen content in exhaust streams produced by combusting air-fuel mixtures in fuel-lean mixture range, considerable effort has been expended in devising an oxygen-sensing element that can provide, in response to fuel-lean mixture exhaust streams, an electrical signal that is both relatively large in its magnitude and its sensitivity. Some of the proposed devices are formed of multiple layers of refractory or ceramic materials that are fused together with glass frits. Some of the proposed devices employ multiple inlet apertures of different sizes that control the flow of gas mixtures into internal volumes where electrodes sense oxygen content. Still other oxygen-sensing elements include combinations of porous and non-porous layers of yttria-stabilized zirconia in combination with multiple electrodes. However, none of these sensing elements are simple to manufacture since they require careful registration and sealing of several thin ceramic layers or sequential, multiple depositions of zirconia and of metal electrodes. Accordingly, a need exists for an oxygen-sensing element that is simple to manufacture and therefore, relatively inexpensive, and that responds to the oxygen content of exhaust streams produced by combusting both fuel-rich and fuel-lean air-fuel mixtures with an electrical signal of relatively large magnitude and of sufficient sensitivity to discern small changes in mixture content.